1. Field of the Invention
The present invention generally relates to a hydrogen storage material. More particularly, the present invention relates to a hydrogen storage material having a novel composition.
2. Discussion of the Background
In discussion that follows, metallic alloy compositions are expressed using atomic ratios, while hydrogen concentrations in the alloys are expressed in weight percent (wt %xe2x80x3).
Various hydrogen storage materials are known, such as, for example, AB5 alloys (e.g. LaNi5), AB2 alloys (e.g. ZrMn2), AB alloys (e.g. TiFe), A2B alloys (e.g. Mg2Ni) and Tixe2x80x94V-based alloys. These alloys are classified as solid solution alloys.
Tixe2x80x94V-based alloys can absorb sufficient hydrogen to form dihydrides. At room temperature and atmospheric pressure, Tixe2x80x94V-based alloys containing dihydrides can also discharge hydrogen to form monohydrides. In contrast, AB5 alloys (e.g. LaNi5) can only absorb sufficient hydrogen to form monohydrides. Compared with the other hydrogen storage materials, Tixe2x80x94V-based alloys exhibit high rechargeability. As a result, Tixe2x80x94V-based alloys are expected to find practical use as hydrogen storage materials.
Practical hydrogen storage materials are required to exhibit high rechargeability, high activation performance and pressure-composition isotherms (PCT curves) with flat plateau regions.
Tixe2x80x94V hydrogen storage materials need high temperature and high hydrogen pressure for activation. In addition, as a result of spatial fluctuations in Ti concentration, Tixe2x80x94V hydrogen storage materials typically show little flatness in the plateau region on their PCT curves. Typical Tixe2x80x94V hydrogen storage materials must have higher hydrogen rechargeability for practical use. Tixe2x80x94V-based alloys containing elements such as chromium (Cr) or manganese (Mn), which have atomic diameters  less than 95% of that of vanadium, can be activated at lower temperature than materials not containing such smaller elements. A homogenization by heat treatment is effective to improve the flatness of the plateau region.
Tixe2x80x94V-based alloys form vanadium-based solid solutions in broad composition ranges. During the alloying process, a concentration gradient of Ti appears along the solidification direction. This causes the plateaus on the PCT curves to show little flatness, because the dissociation pressure of Tixe2x80x94V-based alloys decreases with increasing Ti concentration. To improve the flatness of the plateaus on the PCT curves, homogenization by heat treatment is effective.
Japanese Laid-open Publication No. 10-110225 discloses Vxe2x80x94Tixe2x80x94Cr alloys with a microstructure attributed by spinodal decomposition in body-centered cubic (BCC) phase of ternary alloys. This reference shows a method of controlling the rechargeability of hydrogen by controlling of alloy microstructure. Practically, the disclosed microstructure decreases the activation temperature of the alloy and increases hydrogen discharge capacity.
Japanese Laid-open Publication No. 7-252560 discloses Ti100xe2x88x92xxe2x88x92yxe2x88x92zCrxAyBz. In this formula, xe2x80x9cAxe2x80x9d is at least one element is selected from V, Nb, Mo, Ta and W; and xe2x80x9cBxe2x80x9d is at least two elements are selected from Zr, Mn, Fe, Co, Ni and Cu. The alloy is composed of at least five elements and has a BCC structure. The flatness of the plateau region is improved by controlling the amount of element xe2x80x9cBxe2x80x9d to within the range of 0 less than z less than 20. By controlling the amount of B, the dissociation pressure of the hydrogen storage material can also be controlled.
However, these known hydrogen storage materials have little flatness in the plateau regions of the PCT curves and little rechargeability capacity. In order to improve the flatness and capacity, homogenization by heat treatment is conducted. However, Tixe2x80x94V- based alloys are easily oxidized during heat treatment, even in vacuum, which decreases hydrogen discharge capacity.
The present invention provides a hydrogen storage material that includes at least titanium and nickel in a matrix phase having a body-centered cubic crystal structure. The concentration of the titanium and the nickel in the matrix phase exhibit a positive correlation. The hydrogen storage material according to the present invention can be easily activated at low cost, because the material exhibits flat plateau regions in its PCT curves without homogenization.